Solving Problems by Querying XML - Joins With Many Members
(Page 4 of 4 )

If one of the sets of elements to be joined has a large number of members, then consider usingxsl:keyto improve performance:

  <xsl:key name="part-city" match="part" use="@city"/>

  <xsl:template match="/">
    <result>
      <xsl:for-each select="database/suppliers/*">
        <xsl:variable name="supplier" select="."/>
        <xsl:for-each select="key('part-city',$supplier/@city)">
        <colocated>
          <xsl:copy-of select="$supplier"/>
          <xsl:copy-of select="."/>
        </colocated>
        </xsl:for-each>
      </xsl:for-each>
    </result>
  </xsl:template>

Each stylesheet produces the same result:

  <result>
    <colocated>
      <supplier id="S1" name="Smith" status="20" city="London"/>
      <part id="P1" name="Nut" color="Red" weight="12" city="London"/>
    </colocated>
    <colocated>
      <supplier id="S1" name="Smith" status="20" city="London"/>
      <part id="P4" name="Screw" color="Red" weight="14" city="London"/>
    </colocated>
    <colocated>
      <supplier id="S1" name="Smith" status="20" city="London"/>
      <part id="P6" name="Cog" color="Red" weight="19" city="London"/>
    </colocated>
    <colocated>
      <supplier id="S2" name="Jones" status="10" city="Paris"/>
      <part id="P2" name="Bult" color="Green" weight="17" city="Paris"/>
    </colocated>
    <colocated>
      <supplier id="S2" name="Jones" status="10" city="Paris"/>
      <part id="P5" name="Cam" color="Blue" weight="12" city="Paris"/>
    </colocated>
    <colocated>
      <supplier id="S3" name="Blake" status="30" city="Paris"/>
      <part id="P2" name="Bult" color="Green" weight="17" city="Paris"/>
    </colocated>
    <colocated>
      <supplier id="S3" name="Blake" status="30" city="Paris"/>
      <part id="P5" name="Cam" color="Blue" weight="12" city="Paris"/>
    </colocated>
    <colocated>
      <supplier id="S4" name="Clark" status="20" city="London"/>
      <part id="P1" name="Nut" color="Red" weight="12" city="London"/>
    </colocated>
    <colocated>
      <supplier id="S4" name="Clark" status="20" city="London"/>
      <part id="P4" name="Screw" color="Red" weight="14" city="London"/>
    </colocated>
    <colocated>
      <supplier id="S4" name="Clark" status="20" city="London"/>
      <part id="P6" name="Cog" color="Red" weight="19" city="London"/>
    </colocated>
  </result>

Please check back next week for the continuation of this article.

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